Why does my guitar hum when I turn the volume down?

[Mattnezz]

The buffer is fairly lineair, any coloration is mainly based on the high Z_in (approximated by R_gate ) and low Z_out(depending on fet a few hundred ohms). If you tie the gate-resistor to ground, (or leave is out completely, ie tie the vol-wiper to gate) the negative portion of the signal might suffer from a little less headroom. I'm not sure, but think currentdraw is approx I_DSS , and you might improve on negative going headroom by reducing the source-resistor. If this works fine, part-count might be as low as 3 parts, jfet, R_source, C_output.
Another way to improve headroom is adding an input cap and voltage divider to the gate (partscount+3) so the source would sit at V+/2.
This allows a bigger R_source and I think it might reduce idle currentdraw. Again, not sure, but reason tells it might be around V+/2 divided by R_source.

The tillman is known for it's non linear, (nice) valve-like characteristics. Though subtle (without source-bypass-C), it definitely has a 'sound'. It adds some gain at the cost of slightly higher Z_out. (approx R_drain). This higher (about 6k~7k ohm) Z_out might also be of interest, since it is low enough (about the same as a guitar with the volume completely open), but not so low as to make a classic fuzz sound like a harsh piece of shite from hell.
It clips nice, but it clips at app 2.5V_pp and with some pickups this might not be enough.
Current-draw is about half a mA.

The circuits are both tiny, and can easily be made switchable. In guitar-electronics the science is rather a guide to remain within specs and keep burnt parts to a minimum, the sound is what matters most in the end. It might be worth the extra mA's (some prefer class A over AB-amps) or parts and as always, sound is a personal matter. To put it short... build... experiment...

[thehallofshields]

The tillman is known for it's non linear, (nice) valve-like characteristics. Though subtle (without source-bypass-C), it definitely has a 'sound'. It adds some gain at the cost of slightly higher Z_out. (approx R_drain). This higher (about 6k~7k ohm) Z_out might also be of interest, since it is low enough (about the same as a guitar with the volume completely open), but not so low as to make a classic fuzz sound like a harsh piece of shite from hell.
It clips nice, but it clips at app 2.5V_pp and with some pickups this might not be enough.
Current-draw is about half a mA.

This is great info. Thank you!

Is there non-linear behavior before it approaches the clipping threshold?